Neutral dissociation of methane by electron impact and a complete and consistent cross section set

We present cross sections for the neutral dissociation of methane, in a large part obtained through analytical approximations. With these cross sections the work of Song et al (2015 J. Phys. Chem. Ref. Data 44 023101) can be extended, which results in a complete and consistent set of cross sections for the collision of electrons with up to 100 eV energy with methane molecules. Notably, the resulting cross section set does not require any data fitting to produce bulk swarm parameters that match with experiments. Therefore consistency can be considered an inherent trait of the set, since swarm parameters are used exclusively for validation of the cross sections. Neutral dissociation of methane is essential to include (1) because it is a crucial electron energy sink in methane plasma, and (2) because it largely contributes to the production of hydrogen radicals that can be vital for plasma-chemical processes. Finally, we compare the production rates of hydrogen species for a swarm-fitted data set with ours. The two consistent cross section sets predict different production rates, with differences of 45% (at 100 Td) and 125% (at 50 Td) for production of H₂ and a similar trend for production of H. With this comparison we underline that the swarm-fitting procedure, used to ensure consistency of the electron swarm parameters, can possibly degrade the accuracy with which chemical production rates are estimated. This is of particular importance for applications with an emphasis on plasma-chemical activation of the gas.

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techReport Neutral dissociation of methane by electron impact and a complete and consistent cross section set D.D. Bouwman (Dennis), A. Martinez (Andy), B.J. Braams (Bastiaan) and U. Ebert (Ute)